Magcargo, The Lava Pokémon. The shell on it's back is just skin that has cooled and hardened. It breaks easily with a slight touch. It's shell sometimes spouts intense flames that circulate through its body. It's body is as hot as lava. It returns to it's original size by dipping itself in magma. It's body temperature is about 18,000 Degrees. Water is vaporized on contact. If it's caught in the rain, the raindrops immediately turn into steam causing fog around it.

Battle Moveset

A Good Moveset for Magcargo would have to be this:

Attacks:

Acid Armor
Toxic
Flamethrower
Reflect

Items Attached:

Leftovers

Preferred Nature:

Bold

Strategy Using Magcargo

Magcargo, a Pokemon with stats that often make people see this pokemon as useless, often going with Torkoal over Magcargo due to many reasons (due to its ability to learn Curse, etc, allowing it be an effective sweeper while losing almost all of its already bad speed.)
However, don't rule out Magcargo just yet, he has a nice range of moves to learn, especially with the release of Emerald too. However, since that hasn't been released outside of Japan yet, I shall avoid giving a moveset based of the new move tutors, since most of you will not have access to them yet.
Magcargo has poor speed, a 4x weakness to water attacks and doesn't have the special defense to survive water attacks. However, it learns tanking moves, like Acid armor (learnt through breeding). Toxic will provide a steady reduction in your opponent's HP or will trigger a switch, allowing you to set up another toxic.
Flamethrower works well with its 80 base special attack, it also recieves STAB. Reflect is rather a filler move, although this would leave it open for Brick breaker's, a fighting type move, which Magcargo also has a weakness too.
I would also suggest adding rest in their somewhere, but beware special attacks, who could quite easily defeat Magcargo.
As for the annoyer, it can be quite useful with a moveset similar to this: Attract, Body slam, Swagger, Flamethrower.
So overall, Magcargo can be used, but may suffer because of its low speed and low special defense. It could also make an effective Pseudo passer too (Reflect, light screen).

EV Corner:

Max out the EVs in Defense for Magcargo with any remaining in Special Defense & HP

Strategy Against Magcargo

Magcargo' Defenses are above average but it's Physical is much higher so a Special Volley or a Water/Ground Volley should work wonders

Contest Moveset

A good Contest Moveset for Magcargo would have to be this for the Tough Contest best with Impish, Lax, Relaxed or Bold Nature:

Attacks:

Body Slam
Sunny Day
Flamethrower
Earthquake

Items Attached:

Yellow Scarf

Strategy Using Magcargo

If you do the attacks in this following order you should have very few problems; 1st - Body slam, 2nd - Sunny day, 3rd - Flamethrower, 4th - Earthquake, 5th - Body slam

Locations in Games

Ruby/Sapphire/Emerald

Evolve Slugma

Colosseum

Trade from R/S/E/FR/LG

Fire Red/Leaf Green

Mt. Ember Cave (Rock SMash)

Animé Appearences

Magcargo has had a few Animé Appearences. First a trainer was trying to catch a big one. After that, Macey's Slugma evolved into Magcargo after her battle with Ash. Soon after that, Macey uses it in her battle against Jackson of New Bark Town. And after that, Flannery's Slugma evolves into it and she uses it in her battle against Ash. The Rest are Cameos